The California-Mexico border region suffers from high levels of air pollution due to a large and expanding population, industrial growth, transportation and agriculture. In this project, researchers will investigate cross-border transport of pollutants and their impacts on regional air quality and climate through the collection and analysis of field data. The observational component of the work includes ground-based measurements along the California-Mexico border region, utilizing a combination of a central fixed site that will house state-of-the-science instruments to measure gases, aerosols, radiation and meteorological parameters and a mobile laboratory that can measure surface-atmosphere exchange fluxes using eddy covariance. The study will be complemented by measurements provided by Mexican colleagues at the fixed site and several mobile units for criteria pollutants and meteorological parameters. The main scientific objectives are to (1) characterize the emissions from major sources in the California-Mexico border regions, (2) determine the spatial and temporal variability in anthropogenic emissions of greenhouse gases and traditional air pollutants, and (3) elucidate the transport and transformation of these emissions and assess their impact on local and regional air quality and climate. A planned NOAA and California Air Resources Board study will provide additional data to aid in the analysis of these subjects.
The Intellectual Merit of the project is to assess the sources and processing of particulate matter and its precursors in the border region and their effects on regional air quality and climate. The analysis tasks will much better define the interplay of emissions and secondary particle formation, the diurnal evolution of fine particulate matter chemical composition under degraded air quality conditions, atmospheric processing of aerosols, particularly soot-containing particles, and their transport and transformation.
The Broader Impacts of the research include the direct involvement of graduate students and postdoctoral researchers in the tasks, and improved collaboration between U.S. and Mexican scientists and students. Results from the study will be directly presented to California and Mexican Government officials and will inform the development of air quality management strategies to reduce adverse human health, ecosystem, and climate impacts.
Continued population growth and industrial development along the US-Mexico border are expected to strain environmental resources, including air quality. Ozone concentrations in the sister-city pair of San Diego and Tijuana typically violate the health-based standard on numerous occasions per year. Black carbon is of increasing concern because of its impacts on both climate and health. Previous studies have suggested the existence of a sharp gradient in air quality and pollutant emissions across the border in Texas, but air quality, emissions, and atmospheric transport at the California-Mexico border have not been well characterized. The overall goal of this research was to improve the understanding of sources, transport, and impacts of air pollutant emissions at the California-Mexico border. Our specific objectives were to quantify fluxes of carbon dioxide (CO2, principal greenhouse gas), nitrogen oxides (NOx, precursor to ozone and particulate matter), and particulate matter (agent of respiratory irritation, cardiovascular disease, and climate change) from the land surface to the atmosphere; to use these estimates to illuminate uncertainties in current estimates of emissions, known as emission inventories; and to assess emissions and transport of black carbon along the border. As part of a multi-institutional, multi-national field campaign known as Cal-Mex 2010, we used a mobile laboratory to measure concentrations and fluxes of pollutants in Tijuana and San Diego. This effort has produced a novel data set of NOx fluxes in an urban area. CO2 fluxes were similar to those observed in other urban studies, while particle number fluxes were higher, likely due to differences in measurement techniques as well as the possibility that vehicles and industries in the border region are more polluting than those in areas with stricter emissions controls. Observed fluxes of NOx were 1.4-17 times higher than those in the emission inventory, and observed fluxes of CO2 were similar to those in a greenhouse gas emission inventory for the Mexican state of Baja California. Further attention to the accuracy of emission inventories for the border region, particularly for NOx, may be warranted. Black carbon concentrations were more than two times higher, on average, at three sites in Tijuana compared to one site in San Diego. Black carbon and carbon monoxide were strongly correlated in Tijuana, and the ratio of the two was comparable to that in other urban areas. Tijuana’s black carbon emissions were estimated to be 230-890 metric tons per year, 6-23% of those estimated for San Diego. Large uncertainties in this estimate stem mainly from uncertainties in the carbon monoxide emission inventory, and the lower end of the estimate is more likely to be accurate. Patterns in concentrations and winds suggest that black carbon in Tijuana was usually of local origin. Under typical summertime conditions such as those observed during the study, transport from Tijuana into the US was common, crossing the border in a northeasterly direction, sometimes as far east as Imperial County at the eastern edge of California. Results from this research will contribute to the public welfare by informing regulatory policy. This research indicates that emissions on opposite sides of the US-Mexico border differ and that cross-border transport is important. The project has resulted in one paper in the peer-reviewed scientific literature and one submitted for publication; new research infrastructure in the form of a handheld aethalometer to measure black carbon; training of two Ph.D. students and one M.S. student; education of students and the general public in Tijuana about air pollution; and international collaboration between scientists and policymakers in the US and Mexico.
